1. Field of the Invention
This invention relates to a blackening processing method for a radiation image, wherein a blackening process is performed on a region of no interest, which is other than a region of interest, in a radiation image, and an apparatus for carrying out the blackening processing method for a radiation image.
2. Description of the Related Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image having good image quality by use of the processed image signal have heretofore been known in various fields. For example, as such techniques, the applicant proposed various radiation image recording and reproducing systems which use stimulable phosphor sheets.
When a radiation image of an object, such as a living body, is recorded on a recording medium, such as X-ray film or a stimulable phosphor sheet, it is desirable that adverse effects of radiation upon the living body can be kept as small as possible. Also, if object portions not related to a diagnosis, or the like, are exposed to radiation, the radiation will be scattered by such portions to the portion that is related to a diagnosis, or the like, and the image quality will be adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, a collimation plate made from lead, or the like, is often used in order to limit the irradiation field to an area smaller than the overall recording region of the recording medium, such that the radiation may be irradiated only to that portion of the object, the image of which is to be used.
In cases where a radiation image is recorded on a recording medium, such as a stimulable phosphor sheet, by using a collimation plate, an image of an object, or the like, is recorded in a region (i.e., an irradiation field) corresponding to the region inward from the aperture contour of the collimation plate. Also, a region (i.e., a region outside of the irradiation field) corresponding to the region outward from the aperture contour of the collimation plate is not exposed to the radiation. Therefore, an irradiation field contour on the image, which contour corresponds to the aperture contour of the collimation plate, constitutes edge lines in the image.
The region outside of the irradiation field is not exposed to the radiation. Therefore, in cases where the image is a negative image recorded on medical X-ray film, the image density of the region outside of the irradiation field becomes approximately lowest on the image. By way of example, when the medical X-ray film is set on a viewing screen and the transmission image with light produced by a fluorescent lamp is seen, the region having the lowest level of image density becomes the very bright region. Therefore, in particular, the portion of the irradiation field, which portion is close to the region outside of the irradiation field, cannot be seen clearly due to dazzling effects of the bright region outside of the irradiation field. Also, in cases where an electric image signal is detected from the recording medium, on which an image has been recorded within only the irradiation field, and a visible image is reproduced from the image signal and displayed on an image displaying device, such as a cathode ray tube (CRT) display device, the region outside of the irradiation field has the highest level of luminance, and therefore the image within the irradiation field cannot be seen clearly.
Accordingly, in the radiation image recording and reproducing systems, a process for forcibly replacing the image signal values, which correspond to the region outside of the irradiation field, by image signal values representing a high level of image density (or a low level of luminance) is carried out. The process is ordinarily referred to as the blackening process. (The blackening process is described in, for example, Japanese Unexamined Patent Publication Nos. 3(1991)-98174, 10(1998)-63831, and 10(1998)-63832.)
In order for the blackening process to be performed, it is necessary that the irradiation field contour, which serves as the spatial reference for the blackening process, be set. As techniques for setting the irradiation field contour, techniques for manually setting the irradiation field contour and techniques for automatically recognizing the irradiation field contour in accordance with the image signal and setting the recognized irradiation field contour have heretofore been proposed. As the techniques for automatically recognizing the irradiation field contour, for example, techniques have been proposed, wherein an irradiation field contour is determined by utilizing the characteristics in that the irradiation field contour constitutes the edge lines in the image, at which edge lines the image density changes sharply, and finding a portion, at which the value of the image signal changes sharply. One of such techniques for determining the edge lines has been proposed in U.S. Pat. No. 4,967,079. With the technique proposed in U.S. Pat. No. 4,967,079, the edge lines are determined by (a) setting a plurality of radial straight lines, which extend from a predetermined point in the image (e.g., a center point in the image) toward ends of the image, (b) detecting an edge candidate point, at which the different in image signal value is large, from the image signal values corresponding to positions along each of the radial straight lines, a plurality of edge candidate points being thereby detected with respect to the plurality of the radial straight lines, and (c) determining the edge lines in accordance with the thus detected edge candidate points.
In cases where, for example, a protector is fitted to the site of a hip joint, or the like, of an object, and the image of the site of the object is recorded in this state, the image area corresponding to the protector within the irradiation field becomes the state close to the unexposed state. Therefore, as in the cases of the region outside of the irradiation field, the image area corresponding to the protector constitutes a very bright region within the irradiation field. In such cases, it often occurs that the image areas located around the bright image area corresponding to the protector cannot be seen clearly due to the dazzling effects of the bright image are a corresponding to the protector.
Therefore, in cases where the unexposed region due to the protector is located within the irradiation field, besides the region outside of the irradiation field, the unexposed region due to the protector also constitutes a region of no interest, which is other than the region of interest to be seen. Accordingly, it is desirable that both the region outside of the irradiation field and the unexposed region due to the protector be subjected to the blackening process, such that the region outside of the irradiation field and the unexposed region due to the protector may not adversely affect the image seeing. Also, a subdivision image recording operation is often performed, wherein the image recording region of a stimulable phosphor sheet is divided into, for example, two subdivisions which are horizontally adjacent to each other, and the image recording is performed on each of the two subdivisions by the utilization of the collimation plate. In such cases, two independent irradiation fields, which are horizontally adjacent to each other, are located within one radiation image obtained from an image read-out operation performed on the stimulable phosphor sheet. Therefore, in such cases, it is desired that the regions, which are other than the two independent irradiation fields horizontally adjacent to each other, (i.e., the regions outside of the irradiation fields) be taken as the regions of no interest and subjected to the blackening process.
Specifically, the region of no interest, which is to be subjected to the blackening process, is located in various different states within one radiation image. Heretofore, in cases where the region of no interest, which is located in various different states within one radiation image, is to be subjected to the blackening process, the problems occurred in that a complicated technique is required for specifying the region to be subjected to the blackening process, or complicated internal processing is required for eliminating an inconsistency of the technique for specifying the region to be subjected to the blackening process.